Heat shields are used in various forms for sound and heat insulation while taking into account a wide range of installation and operating conditions for instance in the engine bay area or underneath the car along the exhaust line. Above all they are used for protection of temperature sensitive components and systems in order to prevent overheating, e.g. due to heat radiated and generated by the engine and/or the exhaust system.
For such heat shields a wide range of constructions is known, e.g. single-layer or multilayer formed bodies that can be manufactured from different materials. Due to the high thermal load the main materials used today are mainly metal based, for instance steel, aluminium or alloys.
Heat shields are classically made from metal sheet material, mainly steel, alloys, or aluminium, the material being used for the supporting sheet, cover sheet and also for the insulation function. In many cases the main area of the heat shield is embossed to increase the apparent stiffness of the part. The embossing of the sheet metal material can be done using rollers in a continuous process, with a plate press process or with a stamp and die method. To increase the effectiveness of the heat shield and to reduce the space required for the shield, the metal sheet or stack of sheets may be contoured to closely resemble the shape of the outer surface of for instance the exhaust manifold.
Most of the known 3D structure patterns used today as embossments are made of a repeated single form or topographical element such as a ridge or knob. U.S. Pat. No. 6,966,402 discloses a pattern with a plurality of dimples formed in a geometric shape selected from a group consisting of a spherical shape, a pyramidal shape, a conical shape or a trapezoidal shape and where the dimples are distributed in an offset of, or uniform rows and columns or in a randomised pattern. EP 0439046 A discloses a 3D pattern in the form of a diamond shaped cross-hatching pattern, which allows the sheet to be stretched and compressed as needed. Also the use of wrinkling or dimpling is disclosed, for instance a plurality of creases or ridges such as in the shape of corrugations. U.S. Pat. No. 6,821,607 discloses the use of knobs having a draped or folded type structure, which increases the compression resistance for the individual knobs and therefore increases the bending strength of the entire sheet material. WO 2010/112354 shows an example of such an optimised embossment having a plurality of indentations or embossments, whereby all the embossments are protruding towards the same direction normal to the surface of the plain sheet material, defined as the neutral plane n, essentially by the same distance h away from this neutral plane, and whereby the plurality of embossments together form a regular network, whereby essentially each embossment intersects with at least two other embossments to form a junction.
To mount the heat shield around the heat source, generally standard means for fastening are used such as bolt and nuts, or clipping, normally combined with washers or shims. By using these types of fastening methods the heat shield is rigidly fastened to the vehicle. During use of the heat shield—when the vehicle engine is running and/or the vehicle is rolling—the heat shield can vibrate in a frequency dependent manner, for instance related to the speed of the motor, the road surface the vehicle is driving on, the mechanical response of the vehicle suspension system, etc. These vibrations can induce mechanical loads in the fastening area including large bending moments in the material near the connection fastening area. Over the lifetime of the vehicle, these repeated vibrations, at different amplitudes and frequencies, can induce crack development and growth in the heat shield material, eventually leading to large cracks or material failure in the connection areas, this process is termed the fatigue life or durability of the heat shield.
Fatigue loading is the process or phenomenon of repeated cyclic loadings induced on a material or structure, inducing stresses or strains, which are below the quasi-static failure limits of the material. In sheet metal structures such as heat shields, fatigue induced cracks will generally start on the surface of the material, and then propagate along the part surface and also through the thickness of the sheet material. Crack development and propagation are driven by the stress fields existing due to deformation of the material due to vibration loading. Initial surface cracks are difficult to detect visually, but will lead to reduction in stiffness of the material, and a reduction in vibration response of the heat shield. Failure can be influenced by a number of factors including size, shape and design of the component or the condition of the surface or operating environment.
Therefore connections must take into account the tightness and stability required for each heat shield and also the oscillations affecting the location of the installation, in order to prevent damage to the heat shield and/or the undesired detachment of fasteners.
In praxis the area around such structural holes for mounting of the heat shield, but also for passing through and connecting of sensors or cables, embossing is normally absent or eliminated to obtain a flat area to enhance the contact between the means for fastening and the heat shield, thereby enhancing the overall fastening function. Generally, the connection area will be embossed similar to the rest of the heat shield, but during part forming the connection area will be compressed by flat parallel plates and flattened, compressing the embossed topography to nearly or totally flat. Through the use of materials and the construction of the heat shields, the tightening torque of the screws cannot be set excessively high in order to prevent damage to the heat shield.
The cost effective simple solutions, that use screws with disk rings or spring rings (also known as shims or washers), are also limited because they merely allow a reduction of the surface pressure under the bolt area, but do not influence the transition area from the washer to the connection area, which is often a point of crack nucleation and propagation due to bending induced by vibration loading. Also the use of several unconnected individual parts to attach the heat shields naturally increases the assembly effort and production cost.
In automotive production there is a significant demand for lightweight construction using metal sheet components. The use of thinner sheets is essential in achieving weight reduction of heat shields. However by reducing the thickness of the part, the rigidity of the part decreases and susceptibility to vibration-induced stresses will increase the risk of fatigue failure. Susceptibility to fatigue cracks will be most critical in areas where the material is bent, over high-aspect ratio geometries, or where through holes are formed for mounting the heat shield in place.